1. Field of the Invention
This invention relates to magnetic tape recording, and more particularly to apparatus and methods for efficiently rewriting data to magnetic tape.
2. Background of the Invention
For many years, tape storage has offered advantages in terms of cost and storage density compared to other storage technologies, such as disk storage. Typical applications of tape storage include back-up and archival storage applications. These applications typically require a very high degree of reliability when accessing the data on read-back. An important feature to provide this high reliability is the so-called read-while-write operation of the tape drive. During the read-while-write operation, faulty ECC-protected data segments (referred to hereinafter as faulty “code-word interleaves”, or “CWIs”) are rewritten to assure that the data is correctly written during the write process. This feature is important because it significantly improves the reliability of the write operation.
In LTO-5 and previous LTO and enterprise-level tape drive standards, rewrites are performed based on a strategy similar to that illustrated in FIG. 1. As shown in FIG. 1, sets 106a of CWIs 100 are written simultaneously across multiple tracks 102 on the magnetic tape medium 104. When one of the CWIs 100a is faulty (as shown by the CWI 100a with black fill) the entire CWI set 106a is rewritten within the data set 108 a short distance from the initial CWI set 106a where the faulty CWI 100a was detected. For example, where the tape medium 104 includes sixteen tracks 102, a rewrite is performed when at least one of the sixteen CWIs 100 that are written concurrently to the sixteen tracks 102 is faulty. In the illustrated example, all sixteen concurrently written CWIs 100 are rewritten (as shown by the CWI set 106b with grey fill) a short distance from the initial CWI set 106a. 
The rewrite strategy described above suffers from various shortcomings. First, in the event one or more tracks 102 are dead (either permanently or temporarily), the rewrite scheme results in an intolerable rewrite overhead of at least one hundred percent, since each set 106 of CWIs 100 is written at least twice (once for the initial write 106a and once for the rewrite 106b). This results in a dramatic loss of tape cartridge capacity. Second, the rewrite scheme does not preserve spacing properties of ECC-protected CWIs 100. As a result, the error-correction coding (ECC) may operate in conditions that are worse than what it was designed for. Finally, in the case where there are relatively few random faulty CWIs 100a, rewriting all sixteen CWIs 100—most of which are good and need no rewriting—is not efficient. This efficiency gets worse if the number of parallel tracks 102 is increased, such as from sixteen to thirty-two or more.
In view of the foregoing, what are needed are apparatus and methods to more efficiently rewrite faulty data segments (or CWIs) on magnetic tape. Ideally, such apparatus and methods would maintain sufficient spacing between the rewritten data segments so that the ECC will adequately protect the data contained therein. Yet further needed are apparatus and methods that will maintain the rewrite efficiency when the number of tracks on the magnetic tape is increased.